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Old 01-22-2010, 01:19 PM   #1
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Joist sistering question for 20' room


I have a room that is 20' x 21.5' and it has 2x11 lumber for the floor joists. I am getting too much bounce on the floor and am hoping to sister these boards and/or bridge them while I am redoing my basement. My main question is that the floor joists are roughly 16oc but they already have a second joist running right next to them but they are not connected. I was thinking that I was going to add more material to sister these joists but how much would I benefit from just joining these together and adding bridges? The joist may be a stated different sixe but they are measuring 11x1.5". I am wanting to do this tomorrow morining and would appreciate any help at all.

If you would like a picture here is from my theater rebuild post showing the exposed joists. You may have to scroll a little bit.

http://www.avsforum.com/avs-vb/showt...1212462&page=3

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Old 01-22-2010, 03:26 PM   #2
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Joist sistering question for 20' room


If I understand your post correctly, you have 2x12 joists spaced approximately 16 inches OC spanning either 20 feet or 21.5 feet. The joists are apparently in pairs, although the joists are not connected to each other. Your post was unclear, and I could not tell from the photos, whether each joist comes as a pair, or whether only some of them are paired.

Assuming each joist is paired but not connected, the d/L for 40 psf floor loading (pretty typical for a room) would be approximately 1:600, which is more than stiff enough for any type of flooring. So either I did not understand your post, and your joists are in fact not paired, or you have unusually heavy floor loading, or there is something wrong with the joists.

Installing blocking between the joists is not going to make a significant difference in terms of overall stiffness of the floor, and neither is bridging between the joists. Maybe you can add some information, such as the type of room, the type of loading on the floor, and any unusual conditions that may be contributing to the bouncy floor condition.

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Old 01-22-2010, 04:04 PM   #3
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Joist sistering question for 20' room


Each joist is a double board 2x12. They are not connected however. Above it is the living room with just furniture. If someone walks hard/ runs/ or jumps in the area above in the living room you can feel it if you are sitting on the coach more than I would think you could. I know these are not engineered I beams but I would think there would be a way to strengthen the floor without adding a beam and posts. Here is the direct picture more up close. They are paired and the pairs are approximately 16 oc. If my son which is 2 would jump from my fireplace to the floor I could easily feel it 8 feet away on the couch.

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Old 01-22-2010, 04:14 PM   #4
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Joist sistering question for 20' room


Adding some bridging in there, or blocking will help immensely. You might also be able to add some strongbacks in there. It was hard to tell how far the bulk head was into the room but it would be a perfect place to hide a strongback.
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Old 01-22-2010, 04:32 PM   #5
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Joist sistering question for 20' room


What the heck is a strongback?
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Old 01-22-2010, 04:39 PM   #6
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Joist sistering question for 20' room


http://en.wikipedia.org/wiki/Strongback

In your situation it could be a 2x4 or 2x6 that runs perpendicular to the joists and is attached to the bottom side.
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Old 01-22-2010, 05:34 PM   #7
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Joist sistering question for 20' room


http://www.ufpi.com/product/oj/tech/bridging.htm:

Effects of "Strongback" Bridging

Open Joist™ recommends the use of strongback bridging to enhance the performance of floor systems. While such bridging does not affect the strength or structural integrity of a floor system and, therefore is not a required component of Open Joist floor systems, it is strongly recommended because of the positive effect it has on “comfort” factors perceived by individuals on a floor system. ANSI/TPI 1-2002 includes information in the commentary to Section 7.5.2.4 regarding the use of strongback bridging to reduce the effects of differential deflection in floor trusses:
7.5.2.4 Strongbacking is recognized for serving two purposes: reducing floor vibrations and limiting differential deflection. Strongbacking does not, however, contribute to or enhance the strength or structural integrity of the system.
Strongbacks are typically used to control potential vibration problems, as the addition of strongbacks has proven to stiffen the trusses and increase the dampening of transient oscillations. Vibration in a floor joist due to normal human activity (e.g. walking) includes vibration movements from side to side, and while floor sheathing prevents lateral vibration of the top chord, the bottom can still vibrate back and forth. Thus, placing a strongback at the bottom of the floor truss helps control the side-to-side movement at the bottom and improves the overall perceptible performance of the floor. Even when there is a ceiling on the bottom of the trusses, in which case the drywall will reduce lateral movement, the addition of strongbacks can still help to further restrict vibration. It should be recognized that, while it will not affect the structural integrity of the system, cutting, removing or failing to provide such strongback bracing can result in degradation of the floor system’s ability to dampen vibration.

To download one of the details below, right-click or Ctrl-click on the link, then choose “Save Target As” or “Save”.
To view a thumbnail image of the detail, roll your mouse over the download link.

Bridging – Standard - DWG / PDF Bridging – Alternate - DWG / PDF Bracing Under Non-Bearing Parallel Wall - DWG / PDF
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Old 01-22-2010, 05:47 PM   #8
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Joist sistering question for 20' room


So is the consensus that sistering is not needed? I could strongback in the soffit about 4-5 feet into the room and do some blocking in the remainder of the room?
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Old 01-22-2010, 05:52 PM   #9
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Joist sistering question for 20' room


bob22, I looked at those pdfs and they didnt make much sense or fall in line with what the other people are saying that a strongback is. I understand the principles behind it but since I only have 7'6" ceiling I don't want to lose much more especially since I am 6'5"
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Old 01-22-2010, 06:36 PM   #10
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Joist sistering question for 20' room


Blocking will do very much the same thing as strong backs. Basically you are making the floor work together so that when someone jumps up and down on one joist, the load is transferred to several more.
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Old 01-22-2010, 09:07 PM   #11
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Joist sistering question for 20' room


I believe that the previous posts hit the mark in that the double 2 X 12s on 16" centers should easily take the dead loads and live loads of a typical living space. Blocking, bridging and strong backs will help. The other thing you may wish to consider is if the floor surface above is a carpet over a subfloor, then you may wish to remove the carpet and add another layer of plywood. The plywood should be no less than 1/2". This will strengthen the floor and therefore more evenly distribute any loads to multiple joists. If the floor is hardwood or ceramic tile it will likely not be practical. Good Luck with the project.
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Old 01-22-2010, 09:26 PM   #12
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Joist sistering question for 20' room


You have doubled joists, but they are not nailed together! Buy a palm nailer if gun will not fit between. 2-3 rows, 2" in from top and bottom with 16d, 16"o.c. Add solid blocking, cut to fit measuring at the tops, after sister nailing. More on bridging:


http://books.google.com/books?id=DWs...0floor&f=false

Sistering: http://www.finehomebuilding.com/PDF/Free/021184090.pdf

Be safe, Gary

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